1. Field of the Invention
This invention relates generally to the attachment of thin pliable sheet material, such as plastic film, to a support, such as an enclosure frame work. More particularly, the invention relates to an improved sheet gripping assembly for this purpose.
2. Prior Art
As will appear from the later description, the improved sheet gripping assembly of the invention is capable of many diverse sheet gripping applications involving a wide variety of sheet or sheet-like materials. However, the present gripping assembly is intended primarily for use in the construction of a greehouse of the class which comprises plastic film or sheet material covering and supported by an open frame structure. The invention will be described in connection with this particular application.
One existing method of greenhouse construction involves the erection of an open frame structure from pipes, tubes or the like, some of which are installed upright with their lower ends anchored to the ground to form vertical supporting posts or columns and others of which span the upper ends of the posts to form an overhead framework. Over this frame structure are stretched plastic sheets or films to form a weather and wind resistent covering or skin which is transparent to the sun's ultra-violet radiation.
One essential element of such a greenhouse construction is a means for firmly but releasibly securing the plastic film to the frame structure, such that the film will remain firmly in place under relatively high winds and yet may be quickly and easily replaced when necessary. A variety of film attachment techniques have been devised for this purpose. One of the more widely used techniques involves the sheet gripping assembly described in U.S. Pat. No. 3,803,671. Simply stated, this patented sheet gripping assembly comprises a channel-like base which is firmly attached to the greenhouse frame structure and a removable sheet holding member or bar insertable laterally into the channel of the base through an open side of the channel. In use, the sheet holding bar is removed and the plastic film is placed across the open side of the base and then pushed through the open side into the channel to form essentially a fold in the channel. The holding bar is inserted through the open side of the channel into this fold in such a way as to firmly grip the plastic film between the base and holding bar.
Sheet gripping assemblies of this general type suffer from one disadvantage which this invention overcomes. This disadvantage resides in the fact that tension force in the sheet material tends to, and in the case of the existing sheet gripping devices often does, dislodge the sheet holding or gripping bars from the hose channel through its open side. This tension force may result from wind loads on the sheet material or plastic film, thermal deformations in the sheet material, and so on.
Also, in some applications, the sheet material or plastic film has a double wall thickness with entrapped air between the two walls. Thermal expansion of this air creates tension stress in the sheet material tending to dislodge the sheet gripping bar. In addition, air often collects in a pocket between the base and the gripping bar in such a way that thermal expansion of this air packet exerts an outward dislodging force on the gripping bar.
My earlier mentioned copending application Ser. No. 556,695 discloses an improved sheet gripping assembly which tends to avoid the above problems. This invention provides a further improved and simplified sheet gripping assembly which avoids such problems.